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A OPEN PIT MINING AÇIK OCAK MADENCİLİĞİ

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3 HUB-LOCATION PROBLEM<br />

One of the new topics in location problems<br />

is the hub location problem. The first paper<br />

on hub location problem was published by<br />

Toh and Higgins (1985) and it was on the<br />

application of hub location problem in<br />

airlines and airports. A comprehensive<br />

survey on hub location models and problems<br />

can be found in Alumur and Kara 2008.<br />

Suppose that there are N nodes and if each<br />

node can be either an origin or a destination,<br />

then there are N(N-1) origin-destination<br />

pairs of nodes in the network. Notice that the<br />

pair i-j is different from the j-i pair. Figure 4<br />

shows a network with six nodes (Daskin,<br />

1995). Assume that in this network each<br />

vehicle could service five origin-destination<br />

pairs every day, and then with six vehicles,<br />

six nodes could be serviced every day.<br />

hub networks, instead of servicing each<br />

spoke directly, hub facilities concentrate<br />

flows in order to take advantage of<br />

economies of scale. Flows from a spoke are<br />

transported to the hub, and are combined<br />

with flows that have different origins but the<br />

same destination. This fact will reduce the<br />

total cost of transportation.<br />

In multi hub networks, the hub nodes are<br />

completely connected to one another and<br />

each spoke is connected to at least one hub<br />

such as the network shown in Figure 6<br />

(Hekmatfar and Pishvaee, 2009).<br />

Figure 6. Example of a 2-hub network<br />

Figure 4. A fully connected network (6<br />

nodes and 30 origin-destination pairs)<br />

If one of these nodes is set as a hub node<br />

and then connect it to all of the other nodes<br />

(which are called spoke), then there will be<br />

2(N-1) connections to service all the spokes<br />

via a hub node (Hekmatfar and Pishvaee,<br />

2009). This network is presented in figure 5<br />

and node A is supposed to be the hub node.<br />

Figure 5. A hub and spoke network<br />

(6 nodes with 6 origin-destination pairs)<br />

Again, if each vehicle could service five<br />

origin-destination pairs every day, then in<br />

this network, with six vehicles, 16 spokes<br />

could be serviced every day. Thus, with a<br />

fixed transportation capacity, it is possible to<br />

service more spokes within a hub network<br />

than within a fully connected network. In<br />

In multi hub networks the assumption is<br />

that, the hubs are connected through low<br />

cost and high capacity pathways which<br />

causes a discount on the transportation costs<br />

between a given hub pair (Naeem, 2009).<br />

The advantage of using hubs is the gain of<br />

economic profits by establishing more<br />

qualitative paths between the hubs. It should<br />

be noted that, increasing transportation<br />

capacity could decrease transportation costs.<br />

On the other hand, increase of transportation<br />

capacity is not possible on every path,<br />

because traffic of the path will become an<br />

important issue in this case. Introducing hub<br />

nodes can effectively solve this problem. It<br />

means that not only transportation capacity<br />

increases, but also there is a low traffic<br />

problem as compared to the situation where<br />

there is not a hub node. Alumur et. al (2012)<br />

studied the hub location problem in the<br />

presence of uncertainty about the set-up<br />

costs for the hubs and the demands to be<br />

transported between the spokes.<br />

4 MATHEMATICAL MODEL OF HUB-<br />

LOCATION PROBLEM<br />

The hub-location problem involves<br />

locating facilities and hub (or hubs) in a<br />

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